1. Field of the Invention
The present invention relates to satellite communications. More particularly, the present invention relates to acquiring satellite signals.
2. Description of Related Art
When a radiotelephone is turned on in a cellular or satellite radiotelephone environment, it typically must search for and acquire the forward link signal transmitted by a base station. This forward link signal, referred to as the pilot signal in a CDMA system, is continuously transmitted by the base station. The pilot signal is used by the radiotelephone to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the signals from the base station.
In a CDMA radiotelephone system, the radiotelephone cannot transmit until the pilot signal is acquired. This signal, therefore, must be acquired rapidly after the radiotelephone is turned on since a radiotelephone user typically does not want to wait to make a telephone call after turning on the radiotelephone.
In a ground based CDMA cellular system, pilot signal acquisition typically takes only a few seconds. This is largely due to the known proximity of the cell sites to the radiotelephone and the fact that the cell is stationary. The pilot signal, therefore, has a relatively short delay to reach the radiotelephone and the Doppler shift of the frequency of the pilot signal is slight.
Acquiring a pilot signal in a satellite based communication system, however, may take significantly longer since there is a greater frequency uncertainty that must be searched. The frequency uncertainty may be due to the range of possible Doppler shift caused by the rapidly moving low earth orbit (LEO) satellites. Other sources of uncertainty, such as the larger possible round trip time delay of the signal through the satellite can magnify the acquisition problem. There is a resulting need for a method for rapidly acquiring a signal transmitted from a satellite communication system.